This invention relates to a process for the strengthening of lead-antimony alloys and, more particularly, to an extremely rapid heat treatment method which strengthens specially correlated alloys and enables the alloys to be processed on a continuous production line into storage battery grids.
Lead-acid storage batteries have been used for many years as starter batteries for internal combustion engines. Pure lead is a soft material however, and extensive research has developed a number of alloys to provide specific physical properties desired by the battery manufacturers. Antimony is a common alloying material and amounts up to about 11% have been employed to improve the strength and castability of the lead. Unfortunately, antimony, aside from being relatively expensive, increases the water loss of the battery and is of limited use in a maintenance free battery and attempts have been made to decrease the antimony level in lead battery alloys.
U.S. Pat. No. 3,993,480 discloses a low antimony-lead alloy containing, by weight, 0.5-3.5% antimony, 0.01-0.1% copper, 0.025-0.3% arsenic, 0.005-0.1% selenium, 0.002-0.05% tin, the balance lead. Other low antimony-lead alloys are disclosed therein and show, in general, the effect of the different alloying elements, on the properties of the alloy. U.S. Pat. No. 3,912,537 shows a highly castable lead alloy for producing battery grids containing 0.002 to 0.5% selenium, 0.25 to 0.5% arsenic and up to 4.0% antimony. An improved low antimony-lead alloy for use in the manufacturing of grids for maintenance-free storage batteries is disclosed in U.S. Pat. No. 4,158,563 and contains about 1.3-1.9% antimony, 0.05-0.45% arsenic, 0.02-0.5% tin, 0.02-0.09% copper and 0.003-0.012% sulfur. These alloys are stated to have sufficient hardness, good castability and pasteability, excellent corrosion resistance, good grid growth characteristics and a low drossing rate.
While the alloys of the prior art have solved many of the problems with low antimony-lead alloys in cast grids, modern grid technology presents a new obstacle. The conventional method of preparing grids by casting is relatively inefficient. An efficient automated continuous method is now preferred which produces grids by expanding or punching a wrought lead alloy strip as described in U.S. Pat. No. 4,443,918. For example, expanded plates can be obtained by continuously supplying a lead alloy strip, expanding it, pasting the thus produced mesh-like strip, drying it and cutting it to form individual grids. U.S. Pat. Nos. 3,945,097 and 4,271,586 describe methods and machines for making expanded battery plates. The disclosure of the above patents are hereby incorporated by reference.
Although superior in performance in many aspects of battery grid behavior, wrought antimonial leads have been excluded from continuous grid production means. It has been shown in J. Electrochemical Society, Vol. 128, Part II, No. 8, July-December 1981, pages 1641-1647, that grids prepared from such alloys, as worked, are inherently soft and result in short lived batteries although it is indicated that the grids can be hardened to tensile strengths in excess of 6000 psi with very short term heat-treatments. It is noted, for example, in "Lead and Lead Alloys" by W. Hofmann, Springer-Verlag New York, Heidelberg, Berlin, 1970 on page 89 that heat treatments of wrought antimonial lead alloys at 250.degree. C. for as short as 10 minutes provide a hardening reaction. Cited in Hofmann (footnote 239) is an article by Dean et al. entitled "The Lead-antimony System and Hardening of Lead Alloys" which discloses heat treatments as short as 1 minute in an oil bath. Unfortunately, a short term heat treatment does not, by itself, provide sufficient hardening and the need still exists for alloys and a heat treatment method which will provide a hardened material under the time constraint of a continuous production process.
It is an object of the present invention to provide a continuous process for providing high strength antimonial lead strip or battery grids.
It is a further object of the present invention to provide high strength antimonial lead alloys.
Other objects will be apparent from the following description.,